Atmospheric Pressure Photo-ionization (APPI) is a well-known soft ionization mechanism, which induces the formation of electron-ion pairs from molecules upon the absorption of high energy photons without disintegrating the molecules into multiple fragments. The lowest binding energy of an electron to a molecule, also known as the first Ionization Potential (IP), typically lies within the range of 5 to 25 eV. Hence, high energy photons are required to ionize molecules. These photons are normally created by a low-pressure gas discharge lamp that, depending on the gas fill, typically emits intense light between 105 nm (11.8 eV) and 150 nm (8.4 eV). At these wavelengths, light is commonly referred to as vacuum ultraviolet (VUV) because it is absorbed by air.
Documents U.S. Pat. Nos. 5,393,979 and 6,646,444 disclose photo-ionization detectors according to the state of the art.
Typical applications of APPI include trace gas analysis through the combination of APPI with mass spectrometry, ion mobility spectrometry and liquid or gas chromatography. These applications are made possible through the convenient fact that the IPs of most carrier gasses and carrier liquids lie above the photon energy of VUV lamps. They are thus unaffected by a VUV radiation source, because the energy of the radiation is too low to cause ionization of these constituents. For example, the IPs of water (12.6 eV), acetronitrile (12.2 eV), nitrogen (14.5 eV) and helium (23 eV) are all considerably greater than the photon energies of VUV lamps with low pressure gas fills composed of xenon (8.4 and 9.6 eV), hydrogen (4.9 eV), deuterium (10.2 eV), krypton (10.6 eV) or argon (11.8 eV). Most organic molecules on the other hand exhibit IPs between 7 and 10.5 eV, making VUV lamps ideal devices to detect a wide variety of molecular compounds.
VUV lamps are divided in two categories depending on the electric field used to excite the low pressure gasses, i.e. lamps excited with DC electric fields and lamps driven by radio frequency (RF) electric fields. Lamps operated with DC fields comprise a metallic cathode and anode within a gas filled glass enclosure between which a high voltage of around 1 kV is applied to start the VUV generating discharge. These lamps are most commonly used in gas chromatography and tend to create a narrow and focused output beam. On the other hand, lamps driven by RF frequencies comprise a transparent glass enclosure containing the low pressure gas and can be further divided in subcategories depending on the coupling of RF power to the low pressure gas container of the VUV lamp, i.e. capacitively or inductively. RF VUV lamps exhibit higher energy efficiency than DC VUV lamps and are therefore the preferred VUV photon source for portable gas detectors. In contrast with DC lamps, they tend to emit an unfocussed collimated light beam.
Common silicon based glasses do not transmit VUV. Hence, the VUV transmission window is a crucial component of VUV lamps. Magnesium fluoride (MgF2) and lithium fluoride (LiF) crystals are widely used materials for these transmission windows. MgF2 is a material frequently used in infrared (IR) optics, but it also transmits UV light down to 110 nm. MgF2 is also known as a rugged material resistant to chemical etching, laser damage, and mechanical and thermal shock. This makes it the transmission window material of choice for most VUV lamps. One notable exception is the high energy VUV lamp based on argon, which requires LiF crystals as VUV lamp window because a sufficiently high transmission coefficient for light below 110 nm is required. However, LiF has several disadvantages. First of all, LiF is a highly hygroscopic material, i.e. it disintegrates upon contact with highly humid environments. This is a major contributing factor to the short operational lifetimes, sometimes down to 100 hours, of argon based VUV lamps. A second major disadvantage of LiF is the incompatibility of its thermal expansion coefficient with ordinary glass, thus making it very hard to obtain a good seal, which is absolutely necessary to avoid the ingress of contamination or water in the glass enclosure of the lamp.
Continuous APPI effected with the VUV lamps described above results in a gradual reduction of the lamp transmission efficiency of the VUV lamp windows. This is mainly attributed to the adhesion of hydrocarbons to the lamp window [A. I. Vangonen et al., “Methods to increase the transmission of MgFI windows,” Soviet Journal of Optical Technology, vol. 55, no. 11, pp. 672-674, 1988]. Nanometer thick attachments of hydrocarbons are sufficient to cause a significant drop in VUV intensity. This phenomenon can be partly solved by cleaning the lamp window with methanol. However, operation of the VUV lamp in high concentrations of volatile organic compounds can create such an intense deposition of hydrocarbons on the lamp window that cleaning with methanol does not suffice any longer and a treatment with high purity alumina (Al2O3) particles or fine diamond polish becomes necessary. Such cleaning procedures eventually lead to the gradual degradation and eventual destruction of the lamp window.
It is an object of the invention to provide a transmission window which provides prolonged operational lifetime of the VUV gas discharge lamp while maintaining the transmission efficiency in the VUV range. Other objects and advantages of the invention will be explained below.